A case study at the Mojave National Preserve
by James Andre, Director, Sweeney Granite Mountains Reserve
California is celebrated widely among biologists for its unparalleled diversity and high degree of endemism. And no other state in the United States can rival California’s vascular plant diversity. While deserts are commonly portrayed as barren and lifeless, the flora of the California desert adds significantly to California’s overall diversity. At present, approximately 2,450 native vascular plant species have been documented in the California desert, representing 38 percent of the native species in the state. And perhaps no other region of the California desert rivals the plant diversity found in the Mojave National Preserve, the focus of this article.
The Mojave National Preserve (MNP) of eastern San Bernardino County is an expansive area of just under two million acres, twice the size of Joshua Tree National Park. Comprised of 20 mountain ranges, the MNP spans 70 miles from the precipitous Granite Mountains at the south to the Clark Range capped by Rocky Mountain white fir forest in the north. From the west at Soda Dry Lake playa, the MNP extends another 70 miles east to the pristine bunchgrass grasslands of the Castle Peaks region, located at the Nevada Border. Unpopulated, and still unfragmented by development, the MNP lies in the heart of the eastern Mojave Desert, an area of global significance, as it represents perhaps the largest intact ecosystem in the US outside of Alaska.
At present, an impressive 953 vascular plant species have been documented in the MNP. When compared to the renowned species diversity of coastal California, the MNP fares quite well. Mid-elevation alluvial fans in the Granite and New York Mountains support the highest shrub diversity in all of California, and similar overall species diversity (90–120 total species per hectare) to that of the primeval coastal redwood rainforests of Del Norte County. The remarkable richness of the MNP flora is owed in part to its exceptional geologic and topographic diversity, as well as the rapid speciation and diversification of large genera (e.g., Phacelia, Astragalus, Eriogonum, Cryptantha, Gilia).
Though often described as monotonous, the MNP region is in fact a place of great diversity. Elevations range from below 1000 feet near at Soda Dry Lake, to 8,000 feet atop Clark Mountain. Sprawling lava flows such as Cima Cinder Cones support numerous springs and seeps. Knife-edge limestone ranges, such as the Providence Mountains, are home to a suite of endemic plants. And one of the finest dune systems in North American, the Kelso Dunes, punctuates a dynamic ecosystem where natural ecological processes remain intact.
The MNP is also where we find ancient rings of creosote bush (Larrea tridentata), the longest-lived vascular plant in California. It is also where we find California’s shortest-lived vascular plants, such as the ephemeral summer annuals that can germinate and produce viable seed in just three weeks after a July thunderstorm. And to underscore the quality of the MNP ecosystem, naturalized alien species make up only 7% of the flora, compared to 25% for the rest of the California flora. Owing to the many unique habitat types present, the MNP flora is a hotbed for rare plants, with more than 130 listed species. By comparison, Joshua Tree National Park has less than 50 rare plant species
In 1994, I began my tenure as Director of the University of California’s 9000-acre Granite Mountains Desert Research Center, located on the eastern flanks of the Granite Mountains, embedded within the southwest corner of the Mojave National Preserve. With an academic background in plant taxonomy and ecology, and like a kid in a candy store, I immediately initiated a research project to complete a flora of the MNP and surrounding areas. Unlike most of the California desert, the MNP region received considerable attention from early botanists. In particular, the Clark, New York, Providence and Granite Mountains, the highest ranges in the Preserve, have attracted a number of prominent botanists over the past 150 years, including Marcus E. Jones, W.L. Jepson and Robert Thorne. But for the most part, much of the Preserve remained unexplored botanically.
Prior to launching into two decades of extensive field work, I mapped the approximately 15,000 historic vouchers known from the region in 1994 and noticed that nearly all were clustered in the Kelso Dunes and four high ranges. There were 16 other mountain ranges, such as the Cima Cinder Cones, Castle Peaks, Piute Range, and Woods Mountains, where few to no historic vouchers existed. Additionally, most early collections were made in spring, so the collection record did not represent the substantial summer and fall-flowering component of the flora. Over the course of the project, I focused (with help from a small cadre of botany buddies) approximately 9,000 field hours within these geographic and temporal voids, and also continued to survey the higher ranges and Kelso Dunes.
To summarize our findings over the past 20 years, 132 native species have been added to the MNP flora (a 15 percent increase), many of which were found in the “well-studied” areas (e.g, New York and Providence Mountains). Seven of these species are new to science, more than 800 new rare plant occurrences have been added, and noteworthy range extensions continue to be frequently documented. During the course of this effort, I developed the Granite Mountains Desert Research Center Herbarium, which to date houses nearly 7,000 specimens from the flora project, as well as specimens from the entire California desert region.
As was the case at MNP, similar numbers of new discoveries have been found in recent floristic inventories at Joshua Tree National Park and the Whipple Mountains of southeast San Bernardino County. Assuming these local floras are representative of the undocumented status of the entire California desert, from Mexico to Death Valley, we can take home one clear message: the desert remains a floristic frontier. Since the publication of California’s first comprehensive flora 90 years ago (the 1925 Jepson Manual) botanists have documented an additional 300 species in our desert, representing a 14 percent increase. More than 60 percent of these were added in just the last two decades. This resurgent golden age of discovery is by no means as high as the late 1700s and 1800s when the first wave of exploration occurred, but it’s happening now during a time when field collecting and funding for field work has seen a general decline.
What can we expect with additional inventory in the California desert? Based upon fairly conservative assumptions, we can expect another 190–200 native species to be added to the California desert flora by the year 2100. This projection assumes we have reached the peak rate of discovery at about 35 species per decade, and applies a 10 percent decline in the number of additions per decade going forward. Another way of looking at this projection is to consider what it means to our present level of understanding. If 200 species are to be added to the California desert by the end of this century, and it’s reasonable to imagine another 50–100 more in the next century, then we can safely assume that at least 10 percent of the flora is presently undescribed. This is quite humbling, and reminds us that the floristic frontiers are not only places like the Brazilian Amazon or New Guinea, but our own deserts here in California.
The projection of 200 new additions this century also assumes that extinction is not a factor, and that the desert will remain sufficiently protected into the future. However, 1,500 square miles of BLM lands in the California desert are being targeted for industrial-scale solar and wind energy development over the next several years, mostly on undisturbed lands. Barriers to gene flow, altered pollinator guilds, and similar boundary effects expand the sphere of impacts far beyond the footprint of each project. At this unprecedented scale and pace of impacts, most population models portend that extinction events will be likely. So while we have a lot of work to do still to inventory the deserts, time is now of the essence.